Hydrocarbon producing fields typically include a subterranean fluid that is comprised of a mixture of oil, gas and water, wherein the phase relationship between these components are controlled by the pressure, temperature and composition of the fluid. It is desirable to analyze and evaluate these fluids to determine a variety of fluid characteristics of commercial interest to the petroleum industry, such as the type and quality of the fluid within the reservoir. One way to accomplish this is by retrieving a sample of the subterranean formation fluid to the surface and analyzing the fluid to determine its composition using known techniques, such as gas chromatography.
Gas chromatography is a well-known method for identifying the chemical composition of a material sample and has found application in a variety of industries which rely on the identification of chemical compounds, such as the petroleum industry which uses chromatography to identify the chemical composition making up a subterranean fluid to be extracted. The gas chromatography process involves vaporizing and introducing a material sample into a chromatographic column, wherein the material sample is transported through the column by the flow of an inert, gaseous mobile phase, such as nitrogen (N2), hydrogen (H2) or Helium (He). Although the material sample is transported through the column via the carrier gas, the motion of the analyte is inhibited by the adsorption of the analyte molecules onto a stationary phase.
There are at least two well-known types of columns typically in use with gas chromatography systems: packed columns and capillary columns. A packed column contains a finely divided solid support material (eg. diatomaceous earth) which may be coated with a stationary phase, wherein the nature of the coating material is dependent upon the type of materials to be strongly adsorbed. This allows a packed column to be tailored to separate a specific type(s) of compound. A capillary column, on the other hand, has a very small internal diameter (on the order of tenths of millimeters) and the column walls are coated with the active materials. For example, most capillary columns are made of fused-silica with a polyimide outer coating, or stainless steel, and tend to be flexible, allowing for a very long column which can be wound into a small coil.
As such, the rate at which the molecules progress along the column depends upon the strength of the adsorption, which in turn depends upon the type of molecule and the column material. Since each type of molecule has a different rate of progression, the various components of the sample material are separated as they progress along the column and thus reach the end of the column at different times. A detection device is then used to monitor the outlet stream of analytes from the column to determine the amount of analyte exiting the column as well as the time it takes for the analyte to traverse the column. These substances may then be generally identified by the order in which they emerge from the column and by the residence time of the analyte within the column.
Unfortunately however, the retrieval of formation fluids from a subterranean reservoir to the surface may have undesirable consequences. For example, after samples of petroleum fluids are extracted from the earth formation at high temperature and high pressure, they must be brought to the surface, transferred to a transportation vessel and shipped to a distant laboratory for analysis. Changing temperatures and pressures associated with these operations can lead to changes in the fluids, some of which are irreversible. Additionally, leaks in the pressure vessel and transfers between pressure vessels also tend to change the composition of the fluid. These significant and irreversible changes in the fluid characteristics reduce the ability to accurately evaluate the actual properties of the formation fluid.
Another undesirable consequence involving the retrieval of formation fluids from a subterranean reservoir to the surface includes the time and cost involved in running a sampling tool to the formation of interest, retrieving a sample of the fluid within the formation and analyzing the sample of fluid without affecting the integrity of the composition of the fluid. One way to accomplish this involves maintaining the pressure of the formation fluid sample using various apparatus, see U.S. Pat. No. 5,337,822 (1994) to Massie et al., U.S. Pat. No. 5,303,775 (1994) to Michaels et al., U.S. Pat. No. 5,377,755 (1995) to Michaels et al., and U.S. Pat. No. 6,439,307 (2002) to Reinhardt, incorporated by reference herein in their entireties.
One way to accomplish the desired analysis of formation fluids without compromising the integrity of the fluid composition involves the down-hole characterization of formation fluids using borehole chromatography techniques, see U.S. Pat. No. 4,739,654 (1988) to Pilkington et al. and PCT Pat. Appl No. PCT/US01/40372 (2001) to Storm and Richardson. This may be accomplished by disposing a downhole chromatograph within a well bore and by introducing a sample fluid into the chromatograph, wherein the chromatograph may be powered via an umbilical (i.e. a wireline from the surface), a down-hole turbine/alternator power supply or via a battery device. The chromatograph would then analyze the composition of the formation fluid sample and communicate the results to the surface. Unfortunately, however, several problems still exist with current downhole chromatography devices and techniques. For example, while U.S. Pat. No. 4,739,654 (1988) to Pilkington et al. allegedly discloses a downhole chromatography technique, the method and apparatus disclosed therein cannot be implemented in the field, as they suffer from severe defects, namely inadequate gas handling storage and disposal techniques.
One problem involves the handling of the carrier gas. In conventional gas chromatography systems, the carrier gas is a consumable that is typically provided from a high pressure tank, allowed to flow through the column and vented into the atmosphere, wherein the flow rate of the gas through the column must be constant in order to produce interpretable chromatograms. In laboratory and other surface systems, the constant flow rate through the column is ensured by maintaining a constant pressure drop along the column, wherein a gas regulator is used to control the flow line pressure at the high pressure end of the column and the low pressure end of the column is vented at ambient atmospheric pressure. Unfortunately, this type of carrier gas system is not suitable for borehole application for several reasons.
First, the use of consumable gases in wireline, logging while drilling (LWD) or subsea tools is currently undesirable because they require delivery of these gases to shops and depots. Moreover, when tools are required for multiple jobs at remote sites, e.g. offshore platforms, the logistics of delivery become even more troublesome. Second, in the absence of contact with the atmosphere, there is no infinite reservoir for the disposal of the carrier gas once it has traversed the column. Third, current systems and methods are not capable of maintaining a constant pressure gradient along the chromatography column.
Accordingly, it is an object of the present invention to provide a self-contained chromatography system capable of sample analysis in remote locations. It is a further object of the present invention to provide a self-contained chromatography system for down-hole sample analysis. It is yet a further object of the present invention to provide a chromatography system having an improved gas handling system. Furthermore, it is yet another object of the present invention to provide a chromatography technique having an improved pressure regulation means at the outlet of the gas chromatography system.